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(#) Fragments should specify an `id` or `tag`

!!! WARNING: Fragments should specify an `id` or `tag`
   This is a warning.

Id
:   `MissingId`
Summary
:   Fragments should specify an `id` or `tag`
Severity
:   Warning
Category
:   Correctness
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   Initial
Affects
:   Resource files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://developer.android.com/guide/components/fragments.html
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/MissingIdDetector.java)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/MissingIdDetectorTest.java)
Copyright Year
:   2012

If you do not specify an `android:id` or an `android:tag` attribute on a
`<fragment>` element, then if the activity is restarted (for example for
an orientation rotation) you may lose state. From the fragment
documentation:

"Each fragment requires a unique identifier that the system can use to
restore the fragment if the activity is restarted (and which you can use
to capture the fragment to perform transactions, such as remove it).

* Supply the `android:id` attribute with a unique ID.
* Supply the `android:tag` attribute with a unique string.

If you provide neither of the previous two, the system uses the ID of
the container view.

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
res/layout/fragment.xml:7:Warning: This <fragment> tag should specify an
id or a tag to preserve state across activity restarts [MissingId]
    &lt;fragment
     --------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`res/layout/fragment.xml`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~xml linenumbers
&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" &gt;

    &lt;fragment
        android:name="android.app.ListFragment"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" /&gt;

    &lt;fragment
        android:name="android.app.DialogFragment"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:tag="mytag" /&gt;

    &lt;fragment
        android:id="@+id/fragment3"
        android:name="android.preference.PreferenceFragment"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" /&gt;

&lt;/LinearLayout&gt;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/MissingIdDetectorTest.java)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `MissingIdDetector.testBasic`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Adding the suppression attribute `tools:ignore="MissingId"` on the
  problematic XML element (or one of its enclosing elements). You may
  also need to add the following namespace declaration on the root
  element in the XML file if it's not already there:
  `xmlns:tools="http://schemas.android.com/tools"`.

  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;fragment xmlns:tools="http://schemas.android.com/tools"
      tools:ignore="MissingId" ...&gt;
    ...
  &lt;/fragment&gt;
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="MissingId" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'MissingId'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore MissingId ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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